In order to change a direction of light proceeding into or out of a planar light waveguide, the light waveguide is formed with a groove with a slanted surface which defines a deflection mirror. The groove is formed by a process for example disclosed in Japanese Patent Publication No. 10-300961, in which a dicing blade is utilized to cut a surface of the light waveguide to give the groove. The dicing blade utilized in the publication is configured to have a V-shaped cutting end with a nose angle of about 90° or a wedge-shaped cutting end with a nose angle of about 45°. In view of that the light waveguide is processed to give the groove normally having a depth of 100 μm, the dicing blade having the sharp cutting edge is susceptible to being damaged, and also to a fluctuation while being driven to rotate and advance along a straight path, which results in excessive wearing of the blade and therefore making the resulting groove inaccurate. Further, since the dicing blade is placed perpendicular to the surface of the light waveguide, the resulting deflection mirror will be easy to deviate from an intended potion as the dicing blade is caused to vary a cutting depth.